Stainless steels by powder metallurgy

ABSTRACT

This invention relates to the powder metallurgy of dispersion strengthened stainless steel and also to dispersion strengthened precipitation hardenable stainless steels and wrought products thereof. The invention also relates to a powder metallurgy method for producing wrought metal shapes of such steels characterized metallographically by a uniform distribution of dispersoids in both the longitudinal and transverse directions.

United States Patent Benjamin [451 Oct. 10, 1972 [54] STAINLESS STEELSBY POWDER [56] References Cited 72 EZ 00d Be Suff UNITED STATES PATENTSt tan 'amln' 1 R w m 3,085,876 4/1963 Alexander ..75/206 [73] .Assignee:The International Nickel Company, primary Examiner Car1 D f nh New YorkvAssistant Examiner-R. E. Schafer [22] Filed; 25 19 9 Attorney-Maurice L.Pinel [2]] Appl. No.: 852,824 5 ABSTRACT I Related Application Dam Thisinvention relates to the powder metallurgy of [63] continuatiomimpart of709,700, dispersion strengthened stainless steel and also to March 11968, Pat 3,591362 dispersion strengthened precipitation hardenablestainless steels and wrought products thereofi'The in 52 U.S. Cl..29/1s2.s, 29/l82.7 vention also relates to a Powder metallurgy method51 rm. Cl. .1322: 1/00 for producing wrought metal Shapes of Such steels[58] Field of Search..75/206; 29/1825, 182.6, 182.7 characterizedmetallographically by a uniform tribution of dispersoids in both thelongitudinal and transverse directions.

5 Claims, 2 Drawing Figures THE RELATED APPLICATION In theaforementioned related application, Ser. No. 709,700, which isincorporated herein by reference, a. method is disclosed for producing awrought composite metal powder comprised of a plurality of constituentsmechanically alloyed together, at least one of which is a metal capableof being compressively deformed such that substantially each of theparticles is characterized metallographically by an internal structurecomprised of the starting constituents intimately united together andidentifiably mutually interdispersed. One embodiment of a method forproducing the composite powder resides in providing a dry charge ofattritive elements and a powder mass comprising a plurality ofconstituents, at least one of which is a metal which is capable of beingcompressively deformed. The charge is subjected to agitation millingunder high energy conditions in which a substantial portion or crosssection of the 'Charge is maintained kinetically in a highly activatedstate of relative motion and the milling continued to produce wroughtcomposite metal powder particles of substantially the same compositionas the starting mixture characterized metallographically by an internalstructure in which the constituents are identifiable and substantiallymutually interdispersed within substantially each of the particles. Theinternal uniformity of the particles is dependent on the milling timeemployed. By using suitable milling times, the interparticle spacing ofthe constituents within the particles can be made very small so thatwhen the particles are heated to an elevated diffusion temperature,interdiffusion of diffusible constituents making up the matrix of theparticle is effected quite rapidly.

Tests have indicated that the foregoing method enables the production ofmetal systems in which insoluble non-metallics, such as refractoryoxides, carbides, nitrides, silicides, and the like, can be uniformlydispersed throughout the metal particle. In addition, it is possible tointerdisperse alloying ingredients within the particle, particularlylarge amounts of alloying ingredients, e.g., such as chromium, whichhave a propensity to oxidize easily due to theirrather'high free energyof formation of the metal oxide. In this connection, mechanicallyalloyed powder particles can be produced by the foregoing methodcontaining any of the metals normally difficult to alloy with anothermetal.

THE PRIOR ART Generally speaking, stainless steel alloys are produced bythe conventional technique of melting, casting of the molten metal intoan-ingot, and then, after subjecting the ingot to the usual soakingtreatment followed by surface cleaning, hot working the ingot by stagesto the desired shape. Ingots produced as described above may suffer fromseveral kinds of segregation which can have an adverse effect on theforgeability of the ingot. For example, ingots which generally coolslowly, because of their somewhat large cross section, may develop largedendrites and/or segregates, large and non-uniform distribution of grainsizes and also composition segregates along the length and across thewidth of the ingots. While long time soaking at an elevated temperatureis generally empl'oyed'in an attempt to homogenize the metallurgicalstructure of the ingot, the improvement is generally small. Moreover,soaking treatments, have their limitations, depending upon thetemperatures employed, in that grain coarsening can occur which canadversely affect hot forgeability, extrusion, or rolling.

As is well known, alloy compositions in the molten state are veryhomogeneous. However, it is when molten compositions are solidified thatthey tend towards inhomogeneity as determined by temperature-solubilitylaws. Thus, compositions formulated to provide precipitation hardeningmay not always exhibit uniform and/or optimum precipitation hardeningresponse.

Recently attempts have been made to utilize the techniques of powdermetallurgy in overcoming some of the problems. Powder metallurgy isattractive in that new processes have been developed which have madeavailable new techniques for the production of powder metallurgy parts.One development in particular is hydrostatic pressing which enables theproduction of relatively large sized compacts (or powder metallurgyingots) which can then be hot extruded to form desirable wrought metalshapes. Another important development is canned extrusion, wherein acomposition is vacuum sealed in a metal can, e.g., nickel or iron, andthe whole hot extruded to a desired shape. An advantage of the powdermetallurgy approach is that melting is not involved and, thus, theformation of coarse dendrites and segregates (of the kind characteristicof melting and casting) is avoided from the beginning of the powdermetallurgy process to the final production of the article.

However, while coarse dendrites and segregates characteristic of-meltingand casting are avoided, solid state diffusion at elevated-temperatureshad to be relied upon to achieve a homogeneous alloy composition whichpresented a problem depending on the particle size employed. Generally,fine particles are desirable for achieving the desired diffusion betweenelements in as short a time as possible. On the other hand, one of thedisadvantages of working withfine particles is that they tend to be.pyrophoric and also tend to pick up impurities, such asoxygen, from theatmosphere. Another disadvantage of working with this method is that, inproducing precipitationhardening stainless steels, it is difficult toassure optimum precipitation hardening response, due'to the propensityof such precipitation hardening elements as aluminum, titanium andcolumbium to oxidize during the preliminary stages of manufacture.

An example of a powder metallurgy system which has received particularattention recently is the socalled dispersion strengthened metal oralloy. The possibilities of this system came to light with thedevelopment of the Al-Al O system, otherwise referred to as SAP(sintered aluminum powder) investigated by lrmann and others. Thissystem was a natural combination due to the inherent propensity ofaluminum to have a thin film of alumina on its surface. In a materialwith a high surface-to-volume ratio, such as a powder, the surface oxidecan constitute a considerable fraction .of the powder. Thus, it wasfound that when aluminum powder was consolidated by hot pressing andextrusion, the thin oxide film ruptured, which permitted the weldingtogether of the aluminum particles. The wrought structure produced inthis way was characterized by a dispersion of alumina flakes inthealuminum matrix. However, this technique is not applicable to nickeland iron-containing alloys, e.g., stainless steels, as these metals donot form corresponding oxides having good high temperature stability. Inaddition, the oxides of such metals are readily reducible and are notinert as is aluminum oxide.

Other techniques, such as mixing metal powder and dispersoids togetherhave their limitations imposed by the particle size of the metals.Moreover, dispersoids tend to form stringers in the final wroughtproduct resulting from this method. Internal oxidation of powder(nickel, copper, etc.) containing a solute metal like aluminum, silicon,and the like, has its limitations in that the method is generallylimited to simple binary systems, such as Ni-Al, Cu-Al, Ni-Th, Cu-Si,among others. The formation of mixed matrix metal hydrates by aqueousprecipitation followed by the selective reduction of the matrix metalhydrate, e.g., nickel or copper hydrate mixed with thorium hydrate asthe source of dispersoid has its limitations due to the fact that it isdifficult to produce the more complex dispersion strengthened alloys,like stainless steel and, moreover, segregation can occur while handlingthe materials during the wet stage.

The ignition surface coating process involves mixing metal or alloypowders with a liquid solution of a decomposable compound of theintended refractory oxide dispersoid to coat the metal particles with afilm. For example, nickel powder can be mixed with an alcohol solutionof thorium nitrate after which the mixture is dried and pulverized. Themixture is then heated in an inert or reducing atmosphere toconvert thesalt to the corresponding oxide. Again, the need for fine metal powdersin order to achieve close dispersoid particle spacing introduces thefactor of contamination. Furthermore, a liquid solution tends to causesegregation during preliminary preparation since the last of the liquidto evaporate tends to be very rich in the salt it is desired touniformly disperse. Microstructures of wrought metal products producedby this method tend to shown stringers of dispersed oxide. In addition,when using a process like the foregoing, precautions must be taken whendecomposing the salt so that the coated metal particles do notpyrophorically burn up during salt decomposition. Moreover, oxidation ofchromium, in the case of stainless steel, is apt to occur.

It would be desirable to employ dispersion strengthening as an adjunctto precipitation hardening, since it is known that dispersionstrengthened metals, such as thoriated nickel, tend to resist highelevated temperature which normally solution softens precipitationhardenable alloys. However, while improved high temperature propertiesare obtainable with dispersion strengthened alloys, severe practicalbarriers still remain, such as the production on a consistent basis ofhigh quality stainless steel materials essentially devoid of unwantedoxidation and stringers, and the further practical limitation ofproducing a desirable product at a fairly reasonable cost. Most of themethods proposed heretofore were confronted by the problem of avoidingoxidation of the more reactive alloying elements, e.g., chromium andsuch precipitation hardeners as aluminum, titanium, etc., during powdermetallurgy processing.

In addition, there was no certainty with the methods currently proposedof avoiding the formation of stringers of dispersoids within a wroughtmetal stainless steel shape, such as sheet, strip, rod, tubing, plate,wire, extruded structural shapes, and the like. Stringers aredeleterious to structural elements subject to dynamic loading atelevated temperatures in that they provide sites for stressconcentration and can be an important causative factor in the fatiguefailure of structural shapes at elevated temperatures. For the purposeof describing the attributes of the invention, stringers" are defined asa non-uniform concentration of dispersions characterized by alongitudinal pattern in which a plurality of dispersoids appear to beagglomerated or highly concentrated or confined in a long narrow area,with areas adjacent the stringers which appear to be impoverished in thedispersoid. Such non-uniformity tends to cause stress concentrationsunder conditions of dynamic loading which can lead to failure byfatigue. Stringers are not too apparent when a portion of a structuralelement or shape is viewed in transverse section, the dispersoidsthereof appearing as dots. However, stringers are easily discernedmetallographically by examining a wrought metal product in longitudinalsection.

Stringers are not easily avoided by the powder metallurgy methods of theprior art. Although many attempts have been made to produce dispersionstrengthened alloy structures and, in particular, dispersionstrengthened, precipitation hardenable stainless steels exhibiting goodprecipitation hardening response, none as far as I am aware has beenwholly successful prior to the present invention.

It is thus an object of this invention to provide a powder metallurgymethod for producing a wrought dispersion strengthened stainless steelproduct characterized by a high degree of composition uniformity and inwhich the formation of stringers is substantially inhibited.

Another object is to provide a powder metallurgy method for producing awrought, dispersion strengthened, precipitation hardenable stainlesssteel product characterized by optimum precipitation hardening response.

A further object is to provide a powder metallurgy method for producinga wrought, dispersion strengthened, precipitation hardenable stainlesssteel product in which contamination during the early stages ofmanufacture is substantially inhibited due to the nature of the startingpowders employed.

Still another object is to provide a powder metallurgy method forproducing a wrought, dispersion strengthened stainless steel productcharacterized by a uniform distribution of dispersoids in substantiallyany selected area of said product of average diameter ranging up toabout 500 microns in size determined in both the longitudinal andtransverse section.

The invention also provides as an object a powder metallurgy producedwrought, dispersion strengthened stainless steel product characterizedby a high degree of dispersion uniformity in both longitudinal andtransversesection in any selected area of average diameter of up to 500microns, while being substantially free from stringers.

These and other objects will more clearly appear when taken inconjunction with the following description and the accompanying drawing,wherein:

FIG. 1 depicts schematically a ball charge in a kinetic state of randomcollision; and

FIG. 2 is a schematic representation of an attritor of the stirred ballmill type capable of providing agitation milling to produce compositemetal particles in accordance with the invention.

STATEMENT OF THE INVENTION In its broad aspects, the present inventionis directed to the powder metallurgy production of a wrought,consolidated stainless steel product characterized by a substantiallyuniform composition throughout. In its more preferred aspects, theinvention is directed to the powder metallurgy production of wrought,dispersion strengthened stainless steel products, includingprecipitation hardenable products, characterized by a high degree ofdispersion uniformity and absence of dispersoid-free areas of anysubstantial scope in both the longitudinal and transverse cross sectionsand, particularly, in any selected area of average diameter of up toabout 500 microns or higher at a magnification of up to l0,000 times orhigher. Thus, a selected area in the wrought product of average diameterof about 500 microns would show a high degree of dispersion uniformity.

A non-dispersion strengthened product is to be regarded as substantiallyfree from stringers or segrega' tion if it contains less than volumepercent of stringers or of regions exceeding micronsin minimum dimensionin which there is a significant composition fluctuation from the mean,that is to say, a deviation in composition exceeding 10 percent of themean content of the segregated alloying element. The boundaries of asegregated region are taken to lie where the composition deviation fromthe mean is one-half of the maximum deviation in that region.Preferably, the minimum dimension of the region of compositionalfluctuation does not exceed 10 microns. Preferably, also, the proportionof segregated regions is less than 5 volume percent. Indispersion-strengthened products, the segregated regions do not exceedabout 3 microns in minimum dimension and more preferably do not exceed 1micron or even 0.5 microns in minimum dimension. Compositionalvariations on the scale discussed above may, for example, be detectedand measured by electron microprobe examination.

Such uniformity results from the use of a dense, wrought, metalcomposite particle having a highly uniform internal structure. In otherwords, by starting with the foregoing composite particles as thebuilding blocks in producing the wrought metal shape, the high degree ofuniformity of each of the composite particles is carried forward andmaintained in the final wrought product with substantially no stringersin the internal structure. Such an area, if viewed with specialinstruments, such as X-ray scanning techniques, the electron microprobe,etc., would depict metallographically a highly uniform structure. Suchuniformity results from the use of a wrought metal composite particlehaving a highly uniform internal structure to be discussed below.

The wrought composite metal particles which are employed in the startingmaterial are defined in application Ser. No. 709,700 as being made byintegrating together into dense particles a plurality of constituents inthe form of powders, at least one of which is a compressively deformablemetal. In one method, they are intimately united together to formamechanical alloy within individual particles without melting any one ormore of the constituents. By the term mechanical alloy is meant thatstate which prevails in a composite metal particle wherein a pluralityof constituents in the form of powders, at least one of which is acompressively deformable metal, are caused to be bonded or unitedtogether according to one method by the application of mechanical energyin the form of a plurality of repeatedly applied compressive forcessufficient to vigorously work and deform at least one deformable metaland cause it to bond or weld to itself and/or to the remainingconstituents, be they metals and/or nonmetals, whereby the constituentsare intimately united together. By repeated fracture and reweldingtogether of said composite particles, a fine codissemination of thefragments of the various constituents throughout the internal structureof each particle is achieved. Concurrently, the overall particle sizedistribution of the composite particles remains substantially constantthroughout the processing. By observation of the grinding media, e.g.,balls, during processing, it appears that the major site at whichwelding and structural refinement of the product powder takes place isupon the surfaces of the balls.

The process employed for producing mechanically alloyed particlescomprises providing a mixture of a plurality of powdered constituents,at least one of which is a compressively deformable metal, and at leastone other constituent is selected from the group consisting of anon-metal and another chemically distinct metal, and subjecting themixture to the repeated application of compressive forces, for example,by agitation milling as one method under dry conditions in the presenceof attritive elements maintained kinetically in a highly activated stateof relative motion, and continuing the dry milling for a time sufficientto cause the constituents to comminute and bond or weld together andcodisseminate throughout the resulting metal matrix of the productpowder. The mechanical alloy produced in this manner is characterizedmetallographically by a cohesive internal structure in which theconstituents are intimately united to provide an interdisperion ofcomminuted fragments of the starting constituents. Generally, theparticles are produced in a heavily cold worked condition and exhibit amicrostructure characterized closely spaced striations.

It has been found particularly advantageous in obtaining optimum resultsto employ agitation milling under high energy conditions in which asubstantial portion of the mass of the attritive elements is maintainedkinetically in a highly activated state of relative motion. However, themilling need not be limited to such conditions so long as the milling issufficiently energetic to reduce the thickness of the initial metalconstituents to less than one-half of the original thickness and, moreadvantageously, to less than 25 percent of the'average initial particlediameter thereof by impact compression resulting from collisions withthe milling medium, e.g., grinding balls;

As will be appreciated, in processing powders in accordance with theinvention, countless numbers of individual particles are involved.Similarly, usual practice requires a bed of grinding media containing alarge number of individual grinding members, e.g., balls. Since theparticles to be contacted must be available at the collision sitebetween grinding balls or between grinding balls and the wallof the millor container, the process is statistical and time dependent.

One of the attributes of the type of high energy working employed incarrying out the invention is that some metals normally consideredbrittle when subjected to conventional working techniques, e.g., hot orcold rolling, forging, and the like, are capable of being deformed whensubjected to impact compression by energized attritive elements in anattritor mill. An example is chromium powder which was found to exhibitcold workability and compressive deformability when subjected to millingin accordance with the method of the invention. Compressively deformablemetals are capable of exhibiting a true compressive strain (e,) asdetermined by the relationship e, ln (t /t) where ln natural logarithm,2,, original thickness of the fragment and l= final thickness of thefragment, well in excess of 1.0, e.g., 1.0 to 3.0 or even much more.

By the term agitation milling," or high energy milling is meant thatcondition which is developed in the mill when sufficient mechanicalenergy is applied to the total charge such that a substantial portion ofthe attritive elements, e.g., ball elements, are continuously andkinetically maintained in a state of relative motion. For optimumresults, it has been found advantageous to maintain a major portion ofthe attritive elements out of static contact with each other; that isto'say, maintained kinetically activated in random motion so that asubstantial number of elements repeatedly collide with one another. Ithas been found advantageous that at least about 40 percent, e.g., 50percent or 70 percent or even 90 percent or more, of the attritiveelements should be maintained in a highly activated state.

Since generally the composite metal particles produced in accordancewith the invention exhibit an increase in hardness with milling time, ithas been found that, for purposes of this invention, the requirements ofhigh energy milling are met when a powder system of carbonyl nickelpowder mixed with 2.5 volume percent of thoria is milled to providewithin 100, hours of milling and, more advantageously, within 24 hours,a composite metal powder whose harness increase with time is at leastabout 50 percent of substantially the maximum hardness increase capableof being achieved by the milling. Putting it another way, high energymilling is that condition which will achieve in the foregoing powdersystem an increase in hardness of at least about one-half of thedifference between the ultimate saturated hardness of the compositemetal particle and its base hardness, the base hardness being thathardness determined by extrapolating to zero milling time a plot ofhardness data obtained as a function of time up to the time necessary toachieve substantially maximum or saturation hardness. The resultingcomposite metal particles should have an average particle size greaterthan 3 microns, preferably greater than 5 microns and, moreadvantageously, greater than microns.

By maintaining the attritive elements in a highly activated state ofmutual collision in a substantially dry environment and throughoutsubstantially the whole mass, optimum conditions are provided forcomminuting and cold welding the constituents accompanied by particlegrowth, particularly with reference to the finer particles in the mix,to produce a mechanically alloyed structure of the constituents withinsubstantially each particle. Where at least one of the compressivelydeformable metallic constituents has an absolute melting pointsubstantially above about 1,000K, the resulting composite metal powderwill be heavily cold worked due to impact compression of the particlesarising from the repeated collision of elements upon the metalparticles. For optimum results, an amount of cold work foundparticularly useful is that beyond which further milling does notfurther increase the hardness, this hardness level having been referredto hereinbefore as saturation hardness." This saturation hardness istypically far in excess of that obtainable in bulk metals of the samecomposition by such conventional working techniques as cold forging,cold rolling, etc. The saturation hardness achieved in pure nickelprocessed in accordance with this invention is about 477 kg/mm asmeasured by a Vickers microhardness tester, while the maximum hardnessobtainable by conventional cold working of bulk nickel is 250 kg/mm Thevalues of saturation hardness obtained in processing alloy powders inaccordance with this invention frequently reach values between 750 and850 kg/mm as measured by Vickers microhardness techniques. Those skilledin the art will recognize the amazing magnitude of these figures. Thesaturation hardness obtained in powders processed in accordance withthis invention is also far in excess of the values obtained in any otherprocess for mixing metal powders.

As illustrative of one type of attritive condition, reference is made toFIG. 1 which shows a batch of ball elements 10 in a highly activatedstate of random momentum by virtue of mechanical energy appliedmultidirectionally as shown by arrows 11 and 12, the transitory state ofthe balls being shown in dotted circles. Such a condition can besimulated in a vibratory mill. Another mill is a high speed shaker milloscillated at rates of up to 1,200 cycles or more per minute whereinattritive elements are accelerated to velocities of up to about 300centimeters per second (cm./sec.

A mill found particularly advantageous for carrying out the invention isa stirred ball mill attritor comprising an axially vertical stationarycylinder having a rotatable agitator shaft located coaxially of the millwith spaced agitator arms extending substantially horizontally from theshaft. A mill of this type is described in the Szegvari US. Pat. No.2,764,359 and in Perrys Chemical Engineers Handbook, Fourth Edition,1963, at pages 826. A schematic representation of this mill isillustrated in FIG. 2 of the drawing which shows in partial section anupstanding cylinder 13 surrounded by a cooling jacket 14 having inletand outlet ports 15 and 16, respectively, for circulating a coolant,such as water. A shaft 17 is coaxially supported within the cylinder bymeans not shown and has horizontal extending arms 18, 19 and 20 integraltherewith. The mill is filled with attritive elements, e.g., balls 21,sufficient to bury at least some of the arms so that, when the shaft isrotated, the ball charge, by virtue of the agitating arms passingthrough it, is maintained in a continual state of unrest or relativemotion throughout the bulk thereof.

The dry milling process of the invention is statistical and timedependent as well as energy input dependent, and milling isadvantageously conducted for a time sufficient to secure a substantiallysteady state between the particle growth and particle comminutionfactors. If the specific energy input rate in the milling device is notsufficient, such as prevails in conventional ball milling practice forperiods up to 24 or 36 hours, a compressively deformable powder willgenerally not change in apparent particle size. It is accordingly to beappreciated that the energy input level should advantageously exceedthat required to achieve particle growth, for example, by a factor of 5,10, or 25, such as described for the attritor mill hereinbefore. In suchcircumstances, the ratio of the grinding medium diameter to the averageparticle diameter is large, e.g., 20 to 50 times or more. Thus, using asa reference a mixture of carbonyl nickel powder having a Fisher subsievesize of about 2 to 7 microns mixed with about 2.5 percent by volume ofless than 0.1 micron thoria powder, the energy level in dry milling inthe attritor mill, e.g., in air, should be sufficient to provide amaximum particle size in less than 24 hours. A mill of the attritor typewith rotating agitator arms and having a capacity of holding one gallonvolume of carbonyl nickel balls of plus A- inch and minus r-inchdiameter with a ball-to-powder volume ratio of about 20 to l, and withthe impeller driven at a speed of about 180 revolutions per minute(r.p.m.) in air, will provide the required energy level.

The milling time I required to produce a satisfactory dispersion; theagitator speed W (in r.p.m.); the radius, r, of the cylinder (in cm.)and the volume ratio R of balls-to-powder are related by the expression:

where K is a constant depending upon the system involved. Thus, once aset of satisfactory conditions has been established in one mill of thistype, other sets of satisfactory conditions for this and other similarmills may be predicted byuse of the foregoing expression.

When dry milled under these energy conditions without replacement of theair atmosphere, the average particle size of the reference powdermixture will increase to an average particle size of between about 100to 125 microns in about 24 hours. A conventional ball mill loaded withthe same weight of nickel balls and 3 parts by weight of ball-to-powdergenerally accomplishes a mixing of the powders with some incidentalflattening of the nickel powders and negligible change in productparticle size after up to 24 or 36 hours grinding in air.

Attritor mills, vibratory ball mills, planetary ball mills, and someball mills depending upon the ball-topowder ratio and mill size, arecapable of providing energy input within a time period and at a levelrequired in accordance with the invention. In mills containing grindingmedia, it is preferred to employ metal or cermet elements or balls,e.g., steel, stainless steel, nickel, tungsten carbide, etc., ofrelatively small diameter and of essentially the same size. The volumeof the powders being milled should be substantially less than thedynamic interstitial volume between the attritive elements, e.g., theballs, when the attritive elements are in an activated state of relativemotion. Thus, referring to FIG. 1, the'dynamic interstitial volume isdefined as the sum of the average volumetric spaces S between the ballswhile they are in motion, the space between the attritive elements orballs being sufficient to allow the attritive elements to reachsufiicient momentum before colliding. In carrying out the invention, thevolume ratio of attritive elements to the powder should advantageouslybe over about 4 to l and, more advantageously, at least about 10 to I,so long as the volume of powder does not exceed about one-quarter of thedynamic interstitial volume between the attritive elements. It ispreferred in practice to employ a volume ratio of about 12 to l to 50 tol.

By working over the preferred volume ratio of 12 to l to 50 to l on apowder system in which at least one constituent is a cold workablemetal, a high degree of cold welding is generally obtained where thedeformable metal powder has a melting point above l,000l(. In addition,wrought products produced from the powders exhibit highly improvedproperties. Cold welding tends to increase the particle size and, a theparticle size increases, the composition of each particle approaches theaverage composition of the starting mixture. An an indication thatsatisfactory operating conditions have been achieved is the point atwhich a substantial proportion of the product powders, e.g., 50 percentor percent or percent or more, have substantially the averagecomposition of the starting mixture and reach a substantially steadystate particle size.

The deformable metals in the mixture are thus subjected to a continualkneading action by virtue of impact compression imparted by the grindingelements, during which individual metal components making up thestarting powder mixture become comminuted and fragments thereof areintimately united together and become mutually interdispersed to formcomposite metal particles having substantially'the average compositionof the starting mixture. As the particles begin to work harden, theybecome more susceptible to attrition so that there is a concomitantbuilding up and breaking down of the particles and a consequentimprovement of dispersion. The comminuted fragments kneaded into theresulting host metal particle will generally have a dimensionsubstantially less than that of the original metal powders. Refractoryhard particles can be easily dispersed in the resulting particle atinterparticle spacings of less than one micron, despite the fact thatthe starting powder might have been large in size, e.g., 5, 10 or moremicrons. In this connection, the disadvantages inherent in other powdermetallurgy techniques are overcome.

The product powders produced in accordance with the invention have theadvantage of being nonpyrophoric, i.e., of not being subject tospontaneous combustion when exposed to air. Indeed, the product powdersare sufficiently large to resist substantial surface contamination whenexposed to air. Thus, in general, at least about 75 percent of theproduct particles will be 10 microns or 20 microns or greater in averageparticle diameter. The particles generally range in shape fromsubstantially equiaxed to thick flaky particles having an irregularoutline and an average low surface area per unit weight, i.e., a surfacearea not greater than about 6,000 square centimeters per cubiccentimeter of powder. Because the constituents are intimately anddensely united together, there is very little, if any, internal porositywithin the individual product particles. The product particles may havea size up to about 500 microns with a particle size range of about 20 toabout 200 microns being more common when the initial mixture contains amajor proportion of an easily deformable metal, such as an iron groupmetal, copper and similar deformable metals. Individual phases presentin the product particle as comminuted fragments derived from constituentparticles present in the initial powder mixture retain their originalchemical identity in the mechanically alloyed product powder. Theindividual starting constituents can be identified by standard analyticmeans of sufficient sensitivity including, for example, X-raydiffraction, etc. The integrity of the mechanically alloyed productparticles is such that the hardness thereof can usually be determined onthe particles through the use of a standard diamond indenter employed inusual microhardness testing techniques. In contrast thereto, powderparticles loosely sintered or agglomerated together by conventionaltechniques, e.g., ball milling, will usually collapse or fragment underthe influence of a diamond indenter. The composite product powderproduced in accordance with the invention, on the other hand, ischaracterized by a dense, cohesive internal structure in which thestarting constituents are intimately united together, but stillidentifiable. Such composite particles, because of their compositionaluniformity, make excellent building blocks for the production of wroughtmetal products, such as by hot extrusion of a confined batch ofparticles.

When the initial metal particles have melting points of at least aboutl,000l(, substantial cold working of the resulting composite or coldwelded particles is found to result from the reduction in thickness.This cold working effect promotes fracture and/or comminution of thecold welded particles by action of the milling media. Thus, particles oflarger size in the initial mixtures are comminuted or This in size. Coldwelding of particles, both of original particles and cold weldedparticles occurs with accumulation of material'on the particles beingmilled and on the grinding balls. the latter factor contributes todesired particle growth and the overall comminution and/or fracture ofcold welded particles contributes to size reduction of the particles. Asthe dry milling proceeds, the average particle size of the milledparticles tends to become substantially stabilized with a decrease inboth the amount of subsize particles and the amount of oversizeparticles and with continued refinement of the internal structure ofindividual milled particles. Individual components of the powder mixturebeing milled become comminuted and fragments thereof become intimatelyunited together and dispersed through the matrix of the product powder.The net result of the complex milling process is a destruction of theoriginal identity of the metal powders being milled and the creation ofnew composite product powders; however, the original constituents arestill identifiable. The product powder particles comprise comminutedfragments of the initial metal powders welded or metallurgically bondedtogether, with the dimension across the comminuted fragments beingusually less than one-fifth or preferably less than one-tenth theaverage diameter of the initial metal powder from which the fragment wasderived, e.g., less than 10 microns or less than 5 microns or even lessthan 1 micron, e.g., 0.01 or 0.02 or 0.05 or 1 micron. Refractoryparticles included in the initial powder mixture of a stainless steelcomposition become mechanically entrapped in and distributed throughoutthe individual product powder particles in a fine state of dispersionapproximately equal to the minimum dimension of the aforementionedfragments. Thus, the refractory particle interparticle distance is muchless than the particle diameter of the initial metal powder and can beless than 1 micron, in which case there are essentially nodispersoid-free islands or areas and in which the formation of stringersis greatly inhibited.

It is important that the milling process be conducted in the dry stateand that liquids be excluded from the milling environment since theytend to prevent cold welding and particle growth of metal powder andalso prevent inclusion of fine refractory dispersoid materials in theproduct powder. The presence of liquid ingredients in the powder mixturebeing milled, e.g., water or organic liquids such as methyl alcohol,liquid hydrocarbons, or other liquids, with or without surface activeagents such as stearic acid, palmitic acid, oleic acid, aluminumnitrate, etc., effectively inhibits welding and particle growth,promotes comminution of the metal constituents of the mix and inhibitsproduction of composite particles. Moreover, wet grinding tends topromote the formation of flakes which should be avoided. The finecomminuted metal ingredients also tend to react with the liquid, e.g.,alcohol, and the greatly increased surface area resulting inhibitsextraction of absorbed gas under vacuum. Generally, very fine particlestend to be produced which are susceptible to contamination on standingin air or may even be pyrophoric. A virtue of dry milling is that inmany cases air is a suitable gas medium. Alternatively, nitrogen,hydrogen, carbon dioxide, argon and helium and mixtures of these gasescan also be employed. When the inert gases argon and helium areemployed, care should be taken to eliminate these gases from the productpowder mixture prior to final consolidation thereof by powder metallurgymethods. Inert gas media tend to enhance product particle growth and maybe of assistance when powder mixtures containing active metals such asaluminum, titanium, etc., are being milled. Preferably, the millingtemperature does not exceed about 350F., particularly when oxidizableingredients, such as aluminum, titanium, etc., are present in the powdermixture being milled. Generally, the temperature is controlled byproviding the mill with a water-cooled jacket such as shown in FIG. 2.

DETAIL ASPECTS OF THE INVENTION The foregoing procedure is particularlyapplicable to the production of dispersion strengthened stainless steelsstarting with powders having particle sizes ranging from about 2 micronsto microns or even up to 300 microns. The particles should not be sofine as to be pyrophorically active. Coarse particles will tend to breakdown to smaller sizes during the initial stages of dry milling afterwhich particle growth occurs during formation of the composite metalparticle.

As stated hereinbefore, the powder mixture may comprise a plurality ofconstituents so long as at least one of them is a metal which iscompressively'deformable. In order to produce the desired compositeparticle, the ductile metal should comprise at least about 15 percent,or 25 percent, or 50 percent or more by volume of the total powdercomposition. Where two or more compressively deformable metals arepresent, it is to be understood that these metals together shouldcomprise at least about 15 percent by volume of the total powdercomposition.

Stainless steels which can be produced in accordance with the inventionmay have compositions ranging by weight from about 4 percent to 30percent or 35 percent chromium, up to about 35 percent nickel, up toabout 10 percent manganese, up to about 1 percent carbon, up to about 25percent by volume of a dispersoid of a refractory compound, e.g., 0.05to 10 volume percent, and the balance essentially iron in an amount atleast about 45 percent.

A more preferred range is one ranging by weight from about 8 to 20percent chromium, up to about 20 percent nickel, up to about 5 percentmanganese, 0.05 to percent by volume of the dispersoid, up to about 0.25percent carbon, more preferably up to 0.15 percent carbon, and thebalance essentially at least about 55 percent iron.

As will be appreciated, the stainless steel compositions may containother alloying additions, such as up to about 5 percent silicon, up toabout 5 percent molybdenum, up to about 8 percent tungsten, up to about2 percent aluminum, up to about 2 percent titanium, up to about 2percent columbium, up to about 7 percent copper, and about 0.5 to 10volume percent of the dispersoid.

Where a precipitation hardenable stainless steel is desired, thehardener may comprise at least about 0.2 percent by weight of at leastone precipitationhardening element selected from the group consisting ofup to about 2 percent aluminum, up to about 2 percent titanium, up toabout 2 percent columbium, up to 0.4 percent phosphorus, up to 0.25percent nitrogen, and up to about 5 percent copper. Zirconia and aluminaare preferred as the dispersoid in amounts ranging from about 0.5 to 5volume percent at sizes below one micron, although ceria, yttria,lanthana or rare earth oxide mixtures, thoria, etc., are also effective.

Small amounts of other ingredients may be present, such as some sulfurand/0r selenium for free machining, tantalum, etc. The term balanceessentially iron is not meant to exclude the presence of these or othertolerable ingredients.

The invention enables the production of stainless steels containing auniform dispersion of hard phases,.

such as refractory oxides and refractory carbides, nitrides, borides andthe like. Refractory compounds which may be included in the powder mixinclude oxides, carbides, nitrides, borides of such refractory metals asthorium, zirconium, hafnium, titanium, and even such refractory oxidesas those of silicon, aluminum, yttrium, cerium, uranium, magnesium,calcium, beryllium and the like. The refractory oxides generally includethe oxides of those metals whose negative free energy of formation ofthe oxide per gram atom of oxygen at about 25C. is at least about 90,000calories and whose melting point is at least about 1,300C.

One aspect of the invention resides in a powder metallurgy method ofproducing a wrought, dispersion strengthened, stainless steel productcharacterized by a substantially uniform composition throughout and auniform distribution of dispersoid. The method comprises providing abatch of wrought, composite, mechanically alloyed, dense metalparticles, substantially each of said particles being comprised of aplurality of alloyable constituents formulated to a desired stainlesssteel composition and containing up to about 25 volume percent of adispersoid of a refractory compound, at least one of the constituentsbeing a cornpressible metal. The composite particles are characterizedmetallographically by an internal structure comprising said constituentsintimately united and interdispersed, and also characterized by anaverage size such that the surface area per unit volume of particles isnot more than 6,000 square centimeters per cubic centimeter ofparticles. The batch of particles is then hot consolidated to a wroughtmetal shape, whereby the wrought shape is characterized substantiallythroughout by composition uniformity and a high degree of dispersionuniformity in both the longitudinal and transverse directions. Where thecomposition is precipitation hardenable, optimum response toprecipitation hardening is obtained.

Examples of the types of stainless steel that can be produced inaccordance with the invention are given as follows:

TABLE 1 AlSl Nominal Composition type C Mn Si Cr Ni Others AusteniticSteels 201 0.15 max 5.50-7.50 1.0 max 16-18 3.5-5.5 0.25N

max

202 0.15 max 7.5-10 1.0 max 17-19 4-6 0.25N

max

301 0.15max2.0 max 1.0 max 16-18 6-8 302 0.15 max 2.0 max 1.0 max 17-198-10 303 0.15 max2.0 max 1.0 max 17-19 8-10 0.15 min 304 0.08 max 2.0max 18-20 8-12 308 0.08 max 2.0 max 1.0 max 19-21 10-12 309 0.20 max 2.0max 1.0 max 22-24 12-15 310 0.25 max 2.0 max 1.5 max 24-26 19-22 3140.25 2.0 max 2.0-3.0 23-26 19-22 316 0.08 max 2.0 max 1.0 max 16-1810-14 2.0-3.0

321 0.08 max 2.0 max 1.0 max 17-19 9-12 SXC min 347 0.08 max 2.0 max 1.0max 17-19 9-13 10XC min Cbfla MARTENSITIC STEEL 403 0.15 max 1.0 max 0.5max 11.5-13

414 0.15 max 1.0 max 1.0 max 11.5-13.5 1.25-2.5

431 0.20 max 1.0 max 1.0 max 15-17 1.25-2.5

440 0.75-0.95 1.0 max 1.0 max 16-18 0.75 Mo 8 max 440 0.95-1.2 1.0 max1.0 max 16-18 0.75 Mo C max 501 0.1 max 1.0 max 1.0 max 4-6 0.04-0.65

TABLE 11 A181 Nominal Composition type C Mn Si I: Cr Ni Others 405 0.08max 1.0 max 1.0 max 11.5-14.5 0.1-0.3

430 0.12 max 1.0 max 1.0 max 14-18 4301 0.12 max 1.25 max 1.0 max 14-180.15 S

mm 446 0.2 max 1.5 max 1.0 max 23-27 0.25 N

max

NONSTANDARD GRADES 2.25 Mo 3161- 0.06 1.5 0.5 18 13 0.l3P,0.l5S 418 0.170.4 0.3 12.75 2.0 3.0 W Stain- 0.8 Ti lessW 0.07 0.5 0.5 16.75 6.75 0.2A1 17-4PH 0.04 0.4 0.5 16.50 4.25 0.25 Cb 3.6 Cu 17-7PH 0.07 0.7 0.417.0 7.0 1.15 Al Pl-ll5-7 0.07 0.7 0.4 15.0 7.0 1.15 Al Mo 2.25 Mo17-l0P 0.12 0.75 0.5 17.0 10.5 0.28? HMN 0.3 3.5 0.5 18.5 9.5 0.25 P17-4 0.12 0.75 0.5 16.0 14.0 3.0 Cu Cu M0 2.5 M0

The stable refractory compound particles added to the foregoingcompositions are advantageously maintained as fine as possible, forexample below 1 micron and, more advantageously, below 0.5 microns. Aparticle size rangerecognized as being particularly useful intheproduction of dispersion strengthened stainless steel, is 10Angstroms to 1,000 Angstroms (0.001 to 0.1 micron). The amounts ofdispersoid added may range from 0.05 to 10 volume percent or 0.05 to 5volume percent.

In working with metals which melt above 1,000 K, the heavy cold workimparted to the composite metal particle is particularly advantageous inthe production of dispersion strengthened stainless steels. Observationshave indicated that the heavy cold work increases effective diffusioncoefficients in the product powder. This factor, along with the intimatemixture in the product powder of metal fragments from the initialcomponents to provide small interdiffusion distances, promotes rapidhomogenization and alloying of the product powder upon heating tohomogenizing temperatures. The foregoing factors are of particular valuein the production of precipitation hardenable stainless steels wheregood precipitation hardening response is required. Homogenization and/orannealing can be accomplished, for example, during the heating of cannedpowders prior to extrusion.

One of the advantages of formulating compositions in accordance with theinvention is that very little or no oxidation occurs during high energymilling. Generally, the extraneous oxides which appear in the finalconsolidated products are principally those present in the startingmaterial. However, unlike the kind of oxidation which occurs inconventional melting techniques, these extraneous oxides appear as finedispersoids and can be useful as dispersion strengtheners, provided theyare chemically stable and temperature resistant. It appears that a smallamount of mechanically occluded oxygen in a metastable condition occursin powders milled, for example, in a sealed air atmosphere in theattritor mill. This oxygen is finely distributed throughout the milledpowder in amounts of the order of 0.5 percent to 1 percent or more andmay be utilized to provide an oxygen source for interval oxidation toprovide well-dispersed, fine oxide dispersions of metal oxides of metalswhose oxides have a heat of formation at C. of at least 90,000 caloriesper gram atom of oxygen, e.g., lanthanum, zirconium, aluminum, yttrium,etc., milled into the powder in metallic form. 1

Thus, by producing coarse composite metal powders in accordance with theforegoing, particles of substantially uniform composition are providedfrom which wrought metal products can be produced by hot consolidating abatch (e.g., a confined batch) of the particles to a desired shape, suchas by hot extrusion. Each particle is in effect a building blockexhibiting optimum metallographic uniformity, which uniformity iscarried forward into the final product unlike previous powdermetallurgical methods. In other words, in the case of dispersionstrengthened systems, the dispersoid is already fixed uniformly inposition in the particle so that any possibility of stringers forming inthe final wrought product is greatly inhibited.

As illustrative of the use of the invention in producing stainlesssteels the following examples are given:

EXAMPLE I In producing a stainless steel composition, the followingstarting materials were employed: (a) low carbon ferrochrome of aboutminus 200 plus 325 mesh containing percent chromium, 1.01 percentsilicon, 1.35% Si0 0.54% Cr O and the balance essentially iron; (b) highpurity sponge iron of minus mesh; and (c) carbonyl nickel powder ofabout 3 to 5 microns average size. A 900 gram batch was placed in theattritor mill of the type illustrated schematically in H6. 2 comprising10 percent by weight of the carbonyl nickel powder, 27.2 percent byweight of the low carbon ferrochrome and and 62.8 percent of the spongeiron. Two batches were dry milled (16 hours and 48 hours) using aone-gallon volume of nickel pellets or balls of about one-quarter inchin size at a ball-to-powder volume ratio of about 24 to 1 and animpeller speed of 176 r.p.m.

Both powder products had a final average particle size of about tomicrons, the 48-hour powder having a much finer and more homogeneousmicrostructure. The l6-hour powder exhibited an as-milled hardness of785 Vickers which dropped to 381 Vickers after being heated to 1,800F.for about one-half hour and to 324 Vickers, following heating at 1,950F.for about one-half hour. The 48-hour powder, on the other hand,exhibited much higher hardness retention, the asmilled hardness being794 Vickers, which dropped to only 523 Vickers after heating for aboutone-half hour at 1,800F. and to 409 Vickers after a one-half hour annealat 1,950F. After the 1,950F. heating, the internal structure of thecomposite metal particle is homogeneous. It is found that a l-houranneal at about 2,200F. yields a hardness of about 200 to 220 Vickers. Acommercial atomized stainless steel composition has an asreceivedhardness of about 233 Vickers, thus illustrating the high hardness ofthe stainless steel powder provided in accordance with the invention.The compositions of the steel tested are as follows:

Designation Fe Ni Cr Si "1: SiO, C50 31: C

l6-hour Bal. 15.5 16.9 0.21 0.80 0.17 0.05 48-hour Bal. 15.1 17.6 0.260.88 0.17

The 16-hour sample was annealed at 1,950F. for onehalf hour (minus 80mesh), pressed into a compact at 40 tons per square inch to a density ofabout 74 percent of true density to provide a green strength of 1,085p.s.i.

EXAMPLE [1 Another stainless steel composition was produced by drymilling a mix containing 84 grams of carbonyl nickel powder having anaverage particle size of 3 to 5 microns, 341 grams of high purityferrochrome powder containing about 0.1 percent silica and about 70percent chromium of about 120 micron average particle size, and 763grams of high purity sponge iron powder (0.032 percent carbon, 0.115percent silica) of minus 100 mesh particle size. Two such batches weredry milled well beyond the point of saturation hardness for 40 hours inthe attritor mill with an impeller speed of 176 r.p.m. in air at aball-to-powder volume ratio of about 18 to 1. The resulting drymilledpowder product which was very low in combined oxygen and had anaverage particle size of about 85 microns was vacuum sealed by weldingin a mile steel can and was then heated for about 1% hours to about1,900F. and was extruded to rod at an extrusion ratio of about 12.5 tol. The extruded material analyzed, by weight, in addition to iron, 9percent nickel, 20 percent soluble chromium, and 0.09 percent silicon.No silica was detectable in the extruded metal but 2.15 percent byweight, of chromium oxide was found therein by chemical analysis.Micrographic examination of the extruded material revealed that a finelydivided grayish dispersoid, believed to be chromium oxide, was containedtherein in a uniformly distributed state. Tensile and stress-rupturespecimens were machinedfrom the extruded rod. At room temperature, thematerial exhibited a tensile strength of 195,500 p.s.i., a yieldstrength (0.2 percent offset) of 172,300 p.s.i., an elongation of 7.5percent, a reduction in area of 29 percent and a modulus of elasticityof 26.7 X 10 p.s.i. The material had a Vickers hardness of 421 and wasvery slightly ferromagnetic. A life of 44.9 hours with 2.5 percentelongation was obtained by a stressrupture test at 1,200F. and 35,000p.s.i. load while after over 70 hours the material was still unbroken ina test at 1,500F. and 10,000 p.s.i. load. After a 90-hour anneal at2,000F., the material was nonmagnetic and had a Vickers hardness of 390.The properties clearly demonstrated this material was dispersionstrengthened.

EXAMPLE III In producing a dispersion strengthened, precipitationhardenable, wrought stainless steel product of the 17-7 PH typecontaining by weight 0.07 percent carbon, about 0.7 percent manganese,about 0.4 percent silicon, about 17 percent chromium, about 7 percentnickel, about 1.15 percent aluminum, about 2.5 percent zirconia and thebalance essentially iron, the fol-.

essentially iron; and zirconia of about 400 Angstroms average size. A900-gram batch proportioned to yield the foregoing composition is placedin the attritor mill as in Example 1 and the batch is dry milled for 48hours at 176 r.p.m. using a l-gallon volume of nickel pellets or ballsof about one-quarter inch in size at a ball-topowder ratio of about 24to l. The 48-hour milling is employed to assure composite particles ofoptimum uniformity. The foregoing milling conditions will generallyyield dense composite particles having a size falling within the rangeof about 75 to 135 microns.

The powder is removed from the mill and is then passed through an 80mesh screen, following which it is vacuum sealed by welding in a mildsteel can. The canned powder is then heated for about 1% hours to1,900F. and is extruded to rod at an extrusion ratio of 12.5 to 1, theextruded material having approximately the nominal composition of l7-7PH stainless, except for the presence of a highly uniform dispersion offinely divided zirconia (about 400 Angstroms in average size). Theextruded rod is annealed by cooling slowly from about 1,800F. and thenprecipitation hardened by heating to and holding at 875 to 900F. forabout 1 hour and cooling. Thus, the steel is strengthened using thetwo-fold effect of dispersion strengthening and precipitation hardening.However, prior to precipitation hardening, the dispersion strengthenedsteel in the annealed state may be strain hardened and the precipitationhardening added to provide a three-fold hardening or strengtheningeffect.

EXAMPLE IV A special group of two-phase stainless steels is now knownwhich are compositionally adjusted to provide a microstructurecontaining ferrite and either martensite or austenite. These steelscontain 2 percent, preferably 4.5 percent, to 12 percent, nickel, 18percent to 35 percent, preferably 23 to 28 percent, chromium, up to 1.5percent titanium, up to 1 percent vanadium, balance essentially iron. Aseries of l0-kilogram batches proportioned to provide compositionswithin the aforementioned preferred ranges were milled in an attritormill containing 400 pounds of %-il'lCh diameter steel balls with animpeller speed of 182 r.p.m., using charges of carbonyl nickel powder,mesh sponge iron powder, carbon black and 100 mesh ferrochromium powder.Various milling times and various mill atmospheres were employed, andthe powders were analyzed for carbon, oxygen and nitrogen as set forthin the following Table 111:

TABLE III Run Processing No. condition C I; 0 1: N 1 16 hours-sealed air0.057 0.89 0.14 2 16 hours-sealed air 0.054 0.84 0.1 l 3 16hours-nitrogen 0.06 0.47 0.13 4 l6 hours-nitrogen 0.068 0.44 0.09 5 8hours-nitrogen 0.040 0.42 0.07 6 .4 hours-nitrogen 0.050 0.41 0.06 7 2hours-nitrogen 0.054 0.31 0.075 8 l6 hours-dry CO, 0.60 2.50 0.06 9 16hours-dry C0 0.57 1.68 0.07 l0 l6 hours-nitrogen 0.10 0.66 0.07

plus 2 hours-dry CO,

Powders produced in Runs 3 and 4 were coarser than those produced inRuns 1 and 2, while those produced in Run l were finer. The powdersproduced in Runs 8 and 9 were very fine (87 percent less than 44microns). Runs 3 through 7 demonstrate that use of a nitrogen atmosphereof high purity lowered the oxygen content of this type of powder ascompared to that of Runs 1 and 2. Runs 8 and 9 resulted in largeincreases in carbonand oxygen contents of the powder, but the fineparticle size suggested use of such powder for sintering purposes.

Powder from Runs 1 and 2 was easily consolidated by plate rolling inatitanium gettered steel can at 1,900F. A very fine two-phase (ormicroduplex) structure was observed in this material and in similarpowder partially sintered at 2,000F. The structure in both cases wasfiner than that observed in conventionally cast and wrought material ofsimilar analysis.

Although the present invention has been described in conjunction withpreferred embodiments, it is to be understood that modifications andvariations may be resorted to without departing from the spirit andscope of the invention as those skilled in the art will readilyunderstand. Such modifications and variations are considered to bewithin the purview and scope of the invention and the appended claims.

lclaim:

1. As a powder metallurgy article of manufacture, a wrought, dispersionstrengthened, stainless steel shape containing about 4 percent to about35 percent chromium, up to about 35 percent nickel, up to about 10percent manganese, up to about 1 percent carbon, up to about 5 percentsilicon, up to about 5 percent molybdenum, up to about 8 percenttungsten, up to about 2 percent aluminum, up to about 2 percenttitanium, up to about 2 percent columbium, up to about 7 percent copper,up to about 0.4 percent phosphorus, up to about 0.25 percent nitrogen,about 0.05 percent to about 25 percent, by volume, of a refractorycompound dispersoid having a particle size less than about 0.5 microns,characterized by compositional uniformity throughout and by a highdegree of uniformity of dispersoid distribution in both the longitudinaland transverse direction such that the structure thereof contains lessthan 10 volume percent of segregated regions exceeding 3 microns inminimum 2. The powder metallurgy article of manufacture of claim 1,wherein the stainless steel composition ranges from about 8 to 28percent chromium, about 4 to 20 percent nickel, up to about 4 percentmanganese and the balance essentially at least about 55 percent iron.

3. The powder metallurgy article of claim 1, wherein the compositionincludes about 0.05 to 10 volume percent of a refractory compounddispersoid, and containing less than 5 volume percent of segregatedregions exceeding 1 micron in minimum dimension.

4. The powder metallurgy article of claim 3, wherein the stainless steelcomposition is a precipitation hardenable grade containing at least 0.2percent of at least one precipitation hardening element selected fromthe group consisting of up to about 2 percent aluminum, up to 2 percenttitanium, up to about 2 percent columbium, up to about 7 percent copper,up to 0.4 percent phosphorus, up to 0.25 percent nitrogen and 0.05 to 5volume percent of the dispersoid.

5. The powder metallurgy article of manufacture of claim 1, wherein thedispersoid ranges from about 0.05 to 5 volume percent at an averageparticle size of about l0 to 1,000 Angstroms and at an averageinterparticle spacing of less than 1 micron, said wrought stainlesssteel shape being characterized by compositional uniformity throughout,by optimum precipitation hardening response, and by a high degree ofdispersion uniformity substantially free from stringers over anyselected area taken in longitudinal or transverse section of up to about500 microns in average diameter.

* k IF n'ririmrr or hormones:

Patent No. 2 3,696,486 Dated October 10, 1972 inventoflg) J HN STANWOODBENJAMIN It is certified that error appears in the above-identifiedpatent and that said Letters Patent are hereby corrected as shown below:

Col. 3, line -48, for "shown" read -show-. Col. 6, line 55, after"characterized" insert --by--. Col. 8, line 58, for "pages" read--page-. i

Col. 10, line 25, for "a" read --as--.

Line 27, delete "an", second occurrence.

Col. 11, line 45, delete "This" and in res educed; Line 48, for "the",second occurrence, read This-.

Col. 12, line 7, for "or" read --to-.

Col. 14, Tablel, for "AISI Type 304" read O, 08 max 2 0 max 1. O max18-20 8-l2 Col. 15, Table II "Nonstandard Grades", last'column, for

v "2.5 Mo", second occurrence, read -0.5 Cb--.

Line 62, for "interval" read ---internal--.

Col. l6,'li ne 65, delete 0.037" and insert '--0.037.--

under the column heading "%C".

Col. 17 line 24, for "mile" read "mild";

Claim 1, last line, after "minimum" insert --d'imensiom Signed andsealed this 10th day of July 1973.

(SEAL) Attest:

EDWARD M.FLETCHER ,JR. Rene Te tme' er Attesting Officer; ActingCommissioner of-Patencs 5/6 1 7 QE'HMQA'EE ertnrtrwm Patent NO.3,696,486 Dated October 10, 1972 Inventor J OHN STANWOOD BENJAMIN It iscertified that error appears in the above-identified patent and thatsaid Letters ?atent are hereby corrected as shown below:

Col. 3, line 48, for "shown" read show--. Col. 6, line 55, after"characterized" insert -by- Col. 8 line 58, for "pages" read -page-.

Col. 10, line 25, for "a" read as-. I

- Line 27, delete "an", second occurrence.

Col. 11, line 45, delete "This" and insret *--+re'duced--'.

Line 48, for "the", second occurrence, r-ead This--. Col. 12, line 7,for "or" read to----.

Col. 14, Table I, for "AISI Type 304" read O .08 max 2 O max 1. O max18-20 8-l2 Col. 15, Table II "Nonstandard Grades", lasticolumn, for

"2.5 Mo", second occurrence, read -*O.5 Cb--. Line 62, for "interval"read --internal--.

Col. l6,'line 65, delete 0.037 and insert '--0.037.--

under the column heading "%C" Col. 17 line '24, for "mile" read mild--.

Claim 1, last line, after "minimum" insert d'imension Signed and sealedthis 10th day of July 1973.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. Rene Tegtmeyer Acting Commissioner of PatentsAttesting Officer (J/69) J 4 M QE'HFEQATEE @E @QRREQTEQN Patent No.3,696,486 Dated October 10, 1972 Invmtcflg) JOHN STANWOOD BENJAMIN I Itis certified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected se shown below:

Col. 3, line 48, for shown" read -show-. Col. 6, line 55, after"characterized" insert -by- Col. 8, line 58, for "pages" read---page--.

Col. 10, line 25 for "a" read --as.

Line 27, delete "an", second occurrence.

Col. ll, line 45, delete "This" and insret -'--reduced'. Line 48, for"the", second occurrence, r-ead This--. Col. 12, line 7, for "or" read--to-. Col. 14, Table I, for "AISI Type 304" read --O. 08 max 2 O max 1.O max l8-20 8-l2 Col. 15, Table II "Nonstandard Grades", last icolumn,for

"2.5 Mo", second occurrence, read --O.5 Cb- Line 62, for "interval" read--internal--.

Col. 16, line 65, delete 0.037 and insert ---0.037.-

under the column heading "%C".

Col. 17 line 24, for "mile" read --mild--.

Claim 1, last line, after "minimum" insert -dime'nsion? Signed andsealed this 10th day of July 1973.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. Rene Tegtmeyer I Attesting Officer ActingCommissioner of Pacencs

2. The powder metallurgy article of manufacture of claim 1, wherein thestainless steel composition ranges from about 8 to 28 percent chromium,about 4 to 20 percent nickel, up to about 4 percent manganese and thebalance essentially at least about 55 percent iron.
 3. The powdermetallurgy article of claim 1, wherein the composition includes about0.05 to 10 volume percent of a refractory compound dispersoid, andcontaining less than 5 volume percent of segregated regions exceeding 1micron in minimum dimension.
 4. The powder metallurgy article of claim3, wherein the stainless steel composition is a precipitation hardenablegrade containing at least 0.2 percent of at least one precipitationhardening element selected from the group consisting of up to about 2percent aluminum, up to 2 percent titanium, up to about 2 percentcolumbium, up to about 7 percent copper, up to 0.4 percent phosphorus,up to 0.25 percent nitrogen and 0.05 to 5 volume percent of thedispersoid.
 5. The powder metallurgy article of manufacture of claim 1,wherein the dispersoid ranges from about 0.05 to 5 volume percent at anaverage particle size of about 10 to 1,000 Angstroms and at an averageinterparticle spacing of less than 1 micron, said wrought stainlesssteel shape being characterized by compositional uniformity throughout,by optimum precipitation hardening response, and by a high degree ofdispersion uniformity substantially free from stringers over anyselected area taken in longitudinal or transverse section of up to about500 microns in average diameter.